Ewoks, Tree Trunks, and Imperial AT-STs

This brief look at the Imperial Ford Pinto was created using a
high-quality 25fps vidcap of RoTJ. The brightness was lacking, however,
so I have therefore brightened most of the images on this page, and the
filenames are marked to reflect this when it occurs.

In Return of the Jedi, we are treated to the combat skills of
what the Emperor describes to Luke as "an entire legion of my best troops"
on Endor, seen during their crushing defeat at the hands of the Ewoks.
These troops were equipped with biped walker vehicles. In the
non-canon, these are known as
AT-STs, or "All-Terrain Scout Transport", a knock-off of the canon
"All-Terrain Armored Transport" term from the TESB novel, which referred
to
the huge four-legged AT-AT walkers. In spite of
the non-canon denigration of the vehicles, AT-STs are evidently the
premiere small combat vehicle of the Empire, not reconnaissance vehicles
or transports
(how does one do much transporting in a two-seater?). We have witnessed
their frontline combat use at Hoth alongside the larger AT-ATs,
and we see them serve as the primary fighting vehicle in the thick
forests of the Endor moon. In the RoTJ novelisation, they are referred to
as "war wagons", "armored vehicles", and "war-machines" . . . in the
script, they are "giant",
"mighty machines" . . . hardly terms befitting a mere scout. Indeed, the
only scout/recon activity we see is performed on the speeder bikes which
suit the role best, and which are piloted by people identified as scouts.
Nevertheless, I yield to the conventional designation of "AT-ST"
(whatever it would more properly stand for), simply for the sake of easy
identification.

In the midst of the Endor battle, the Ewoks begin
employing their
primitive, pre-industrial technology against the Imperial walkers. Some
of these attempts were unsuccessful, such as those involving dropped or catapulted rocks. Other attempts were literally
"smashing successes". One such effort involved two logs suspended in the
air which, when released, swung into one another, crushing the AT-ST
between them. This total destruction of the vehicle is an interesting
event, worthy of further examination, since it lends itself well to
analysis of Imperial armor.

The above image offers a nice opportunity to perform scaling to
determine the size of the AT-ST. The commander's entire torso is visible,
along with several easily measured areas which can serve as excellent size
reference. In the image, the commander's height from belt to the top of
the helmet is 39 pixels. The height of the chin gunwell is 45 pixels,
width being 49 pixels. The upper, outer corners of the viewport "eyes"
are 76 pixels apart. (Though the front face is angled in this view, it
happens to
measure 139 pixels from the bottom of the outer chin to the top of the
forward face.)

Assuming a standard height of 1.8 meters for the commander (1.9 with
helmet), a likely height from helmet to buckle should be about .8 meters,
especially given the high-riding belts on those uniforms. However, we'll
still give it .9 meters, since such overestimation shall prove
conservative for later uses.

This allows us to get a rough meters per pixel figure at the
commander's range. In this case, it is approximately 0.023 m/px. With
that figure, we may estimate the chin gunwell to be 1.038 meters tall and
1.130 meters wide. The viewport line reads 1.754 meters. (The face is
just under 3.2 meters tall in this image, suggesting a total height of
just over nine meters. Given that the full-scale model of the AT-ST was
built to a height of 8.13 meters, the aforementioned presumption of
overestimation is correct.)

Now that we have a workable estimate for the size of the AT-ST crew
cab, we can now perform scaling work with the logs which destroyed the
AT-ST, determining all that is required to get an estimate of their
kinetic energy.

The AT-ST and logs

The above image
occurs about one frame before the impact of the
left-hand log. The right-hand log has already begun colliding with the
extension on the side. As you can see to the right, the width of the chin
gunwell is 53.3 pixels ('Pythagorized' due to the angle). The top outer
corners of the viewports measure 93.3 pixels. The angle of view evidently
produces some distortion, since getting a meters per pixel figure from the
chin gunwell gives us 0.0212 m/px, whereas the viewports give us a mere
0.0188 m/px. I shall use the larger figure in this instance, and given
that the logs are a bit more distant, I shall bump that figure up slightly
to 0.022 m/px. The diameter-line of the left trunk is at an angle, but is
reached by two corners of a box 67px by 10px. Overestimating slightly, we
arrive at 67.75px for the diameter of the log. At 0.022 m/px, that gives
us a log diameter of 1.49 meters . . . 1.5 for short. Note, of course,
that the log's end is much thicker than the remainder . . . but
nevertheless, the full 1.5 meter figure will be used. Now to the length.
Unfortunately, we never get a perfect shot of the entire log. But, we do
see the lines which suspend it, and can make a reasonable guess. The
distance from the rope to the edge of the trunk
(before the point starts) is 94.7 pixels. At 0.022 m/px, that's a hair
over 2 meters.

The log in flight,image marked for
measurement

The only clear shot of the entire log is in the frame above. I am
assuming that the front and rear anchor points are the same distance from
the edges of the logs, which not only appears to be the case, but also
makes good sense under the circumstances. If so, then the
length of the log is approximately four times the length from edge to
rope. We'll give it 4.5 for safety's sake.

So, we arrive at a log which should be nine meters long. That, with
the diameter, works out to a volume of 15.9 m3.

Now, to figure out the mass, all we need is the density. According to
this
PDF published by the United States Forest Service, a
wood's maximum possible density would seem to be about 1,750
kg/m3. (The
highest I've found mentioned elsewhere for wood that's still "green"
(i.e. with high water content as opposed to the dried-out, lower-density
wood as used for construction) is
from another Forest Service publication, which had a particular species of
oak in the 900 range.)

It should be noted, however, that the density could not have been
extraordinarily high. The trees had to have been able to stand upright
when living, of course, but we must also consider the fact that the Ewoks
weren't exactly working with Kevlar rope. As seen below, a simple stone
axe was sufficient to cut the main cable holding the logs in place, and it
is likely that a similar material was used to suspend the logs. Unless a
mind-boggling array of pulleys was being used, this could place some
severe limits on the mass of the logs.

The release of the logs

A volume of 15.9 m3 at a density of 1,750 kg/m3
gives us a total mass of 27,825 kilograms . . . which we'll just round up
to 28,000 kilograms for ease.

And now, for the kinetic energy, we need velocity . . . we have the
mass.

First frame with left log visible

Impact, 11 frames later

The logs travelled 243 pixels over the course of 11 frames, for an
average speed of just over 22 pixels per frame. At 0.022 m/px, that works
out to .486 meters per frame. The video capture I have is at 25 frames
per second, which gives us a speed of 12.15 meters per second. So:

We can go ahead and round up to 2.1 megajoules.
(With a wood density of 1000 kg/m^3 (which
would seem to be a more common high-end figure), the KE would drop to a
mere 1.2 megajoules.)

And what was the effect of two such log collisions at once? One frame
after the left log's impact, the AT-ST is seen bursting into flames. It
should be noted that the full impact of the logs isn't even complete.

The logs continue on their path, totally crushing the cab as they
appear to meet in the middle:

A few frames later, all that's left is a smashed cabin and burning,
smoking debris.

Note also that the logs rebounded after impact. Apparently, some of
the impact forces
involved appeared to transfer across to the cab to the opposite logs in a
bounce effect, in a manner not unlike the popular swinging
ball desktop toy. Nevertheless, if we assume that the AT-ST's armor
had managed to stop the logs entirely (and thus severely overestimate the
effectiveness of the armor), then we can get a decent
estimate of impact force. The hull was 83 pixels
wide (we'll say 70 for ease and to account for squished metal and troops).
At the aforementioned 0.022 meters per pixel, that's a stopping distance
of 1.54 meters. We therefore end up with an impact force of 1,342,022.72 Newtons. Let's say that the somewhat-pointed tips of the logs were
squares a mere 10 centimeters wide, and assume for the moment that the
point angle is far more acute (in bullet terms, a longer ogive . . . i.e.
that the rest of the log's diameter wasn't about to hit, too).

This works out to a pressure of 134.2 megaPascals. If we go with the
full size of the log, this pressure would drop to 596.5 kiloPascals.

Though the armor was not visibly pierced by the logs, the armor was
severely bent and mangled (along with the structure of the vehicle).
Barring some exotic armor material that is extremely soft and
malleable against logs yet unpiercable by conventional means, we can make
a few comparisons.

1. A .44 Magnum

According to ballistics testing and information websites, a .44 Magnum
bullet weighs in at 15.55 grams. Common muzzle velocity is around 426
m/s. This works out to just under 1411 joules of kinetic energy. If the
primary armor component of the AT-ST is a mere centimeter thick (for
example), it would most likely penetrate. Why? Given the bullet's size
(.429 inches, or 1.09cm), total frontal area should be
0.9331cm2. Thus, the
pressure would be 151,208.7 N/cm2, or 1,512 megaPascals,
over
ten times the figure for the 10cm log.

So how thick is the armor in reality? Unfortunately, we don't know.
In the shot I used and marked to scale the log, we do see the viewport
covers roughly edge-on, and they seem to be no more than 5 pixels thick.
At 0.022 m/px, that's .11 meters, or 11 centimeters. Assuming that's
common for the remainder of the vehicle, the impact force drops to 12,827
newtons, with the pressure dropping to about 13.7 kiloPascals. So, unless
someone fires into the wide-open, window-less viewports of the AT-ST, the
crew is almost certainly safe from handgun fire . . . though it will leave a
mark.

2. A M829A2 "Silver Bullet" Depleted Uranium
Anti-Armor Round

These are tank rounds, fired to excellent effect by M1A1 and A2 Abrams
tanks during the Gulf War. According to this site: "...
the current A2 version throws a slightly longer (30") but skinnier (.8")
10.85 pound dart at 5512 f/s."

To stop that force in 11 centimeters, the armor of the AT-ST would
have to withstand a force of 63,131,956.4 newtons. At .8 inches (2.032cm)
dart diameter, that works out to a frontal area of 3.24 cm2,
for a pressure of 19,485,171.7 N/cm2, or 194,851.7 megaPascals.
I think it safe to say that the AT-ST will be nicely slain by such a
round.

3. 30mm GAU-8/A firing DU Rounds

As per this and FAS.org,
"Depleted
uranium rounds are also fired by a 30-mm, seven-barrel gatling gun mounted
in the nose of the A-10 Thunderbolt aircraft, the only U.S. military plane
that employs depleted uranium rounds. Depleted uranium is the primary
munition for the A-10 Thunderbolt for combat. Each 30-mm depleted uranium
projectile contains approximately 0.3 kg (0.66 lb) of extruded depleted
uranium metal alloyed with 0.75% titanium. The projectile is encased in a
0.8-mm-thick aluminum shell as the final depleted uranium round." A
link from the FAS site offers a DU penetrator diameter of 15mm. At a
reported speed of 3,300 feet per second, we have the following:

KE = .5 (.3) (1,005.8)^2
KE = 0.15 (1011633.64)
KE = 151,745 J

For 11cm armor, that translates to a force of 1,379,500.4 newtons. At
1.5cm DU penetrator diameter (ignoring the remainder of the bullet and
mass for the time being), we thus end up with a pressure of
780,637.5 N/cm2, or 7,806.4 megaPascals. Let's not
even mention the firing rate of the A-10's gun. Suffice it to say
that the AT-ST is toast.

4. A Red and Black Customized 1982 GMC
Van

Previously, I've half-jokingly speculated that based on eyeball
estimates, the AT-ST was no more resilient than a minivan. Well,
whether or not that's the case, it is interesting and amusing to note
that if we were to assume that Mr. T's "helluva-tough", "helluva-fast"
customized 1982 GMC Van only had the weight of a modern Chevy Lumina
minivan (a little plastic thing weighing in at 2325 kilograms)
and that it was travelling at 100 mph (160.9 km/h, or 44.7 m/s),
it would have a kinetic energy of 2.3 megajoules, .2 megajoules more than
an individual log, and impact would occur over a similar area. Of course,
2325kg is only the weight of Mr. T's gold chains, so your figures may vary.

And yes, he would likely hit the legs, which are probably somewhat
tougher than the head. Some have suggested that the head would be very
light for weight distribution, but this is not necessary. We bipeds carry
a great deal of our weight above the legs, as do most living quadrupeds.
It is not the most stable configuration possible, but it serves us quite
well for mobility. Further, the slip and fall of the AT-ST on the logs
demonstrates that stability is not exactly an AT-ST specialty. But
we'll come back to that shortly . . . for now, I'll simply point out that such
a collision would almost certainly result in a knock-down.

Phasers are a funny thing. They can stun people, heat rocks, melt
things, blow things up, and vaporize them without actually producing
vapor. Nevertheless, we do have a canon statement regarding their power:

In "The Mind's Eye"[TNG], Data and Geordi are in Engineering testing a
recovered phaser rifle. Though it turns out to be a copy of a
Federation rifle and of Romulan origin, the following energy usage comment
by Data illicits no apparent suspicion:

A few seconds later, Data mentions that the normal phaser
discharge crystal fires with 86.5% efficiency, which (for
example) could be taken to imply a firepower of .91 megajoules
per second. According to DITL,
the weapon is fired for a total of 51 seconds. We do not know if the
stated draw is the maximum of the phaser rifle, nor are we told the actual
output. On the other hand, we know that hand phasers can vaporize human
beings. Just to vaporize a single kilogram of water at 37 degrees Celsius
requires 2,764,600 joules (2.7 megajoules), so we know that the effective
output of a phaser should be in this range, at least. In TOS, TNG, and
Insurrection, we've seen phasers used to heat rocks to glowing or
blow them apart, and
we've been told of thousands-of-degrees temperatures being created by
phasers. Even at .91 megajoules, though, and assuming a 3cm2
beam spread, we're still looking at 3000 newtons per centimeter, or 30
megaPascals as a possible phaser discharge level upon striking armor.
This compares rather favorably to the AT-STs kinetic armor potential,
though its armor may have energy dissipation abilities we don't know
about. It is quite likely, however, that a phaser would be more than
sufficient to defeat an AT-ST, especially when the more energetic phaser
capabilities are considered.

The Fall

Of course, some have claimed that phasers are
ineffective against any
dense, tough materials. This claim, though absurd, is rather common in
Warsie circles. Even if we were to benevolently grant the claim some
validity, however, AT-ST armor is most decidedly not an uber-dense
material. Behold the behavior of an AT-ST that merely falls over after
slipping on a few logs:

Note that upon impact, the entire head section warps considerably
before the entire vehicle explodes in flames. Even the guns,
vertical in the last image above, have bent. This hardly constitutes
proof of wondrously advanced or dense materials. Further, the designers
evidently
didn't even know that armor set at an angle gives an artificial but
apparent thickness boost. Instead, all the armor is set in a nice flat box,
right where it can be shot clean through, if shooting (as opposed to
tripping, mentioned in the novel) were even required.

In short, the AT-ST is, for all intents and
purposes, the Galactic
Empire's most embarrassing ground combat vehicle. They are, as the novel
states, "clumsy armored vehicles", and extremely fragile ones at that. If
we assume that the Emperor's best troops got his best equipment, then it
becomes even more embarrassing.

It should also be noted that, as a matter of general principle, one's
land-based combat vehicles are usually tougher targets than one's
aircraft. Of course, in the repulsor-lift-capable Empire this may not be so but, if it is the case, then TIE fighters are tin cans of death.

Some objections appeared to this page, and they were conveniently collected
by Mike Wong and Mike Blackburn and employed all at once. The response,
part of the "Battle of Britain" site attack response, appears here.

Special thanks to T. Lee for corrections regarding modern military
weapons.